The present invention generally relates to pretreatment of fuel cell feed airstreams and specifically to a device and method for humidifying an airstream to a fuel cell using porous carbon foam.
Conventional methods for humidifying air involve spraying water over a high surface area medium (cloth, steel wool, etc) and forcing the dry supply air over the moist medium, which results in evaporation of the water from the surfaces of the medium, thereby producing humid air suitable for the supply of a fuel cell. However, the drawbacks to these conventional methods are that the evaporation of the water from the evaporating medium produces an endothermic effect and the medium chills dramatically, albeit very slowly due to its low conductivity. The results of this cooling effect is that the supply air cools and reduces the saturation point of the air (which results in a lower humidity content once the air is heated going into the fuel cell) and the cooled evaporating medium and water present then have a lower thermodynamic driving force to evaporate. Attempts to overcome this by heating the evaporative medium have been unsuccessful since the traditional evaporative mediums exhibit low thermal conductivities, which result in high losses and low efficiency of supplying the heat of vaporization to the water/medium. If the medium is a cheap steel wool or cloth fabric, the thermal conductivity can be as low as 1 watt per meter per degree Kelvin (W/m·K). If the medium is an expensive aluminum or copper foam (which is not the traditional choice), the thermal conductivities are not much better at about 10–20 W/m·K. Conversely, by utilizing a high conductivity graphite foam, the thermal conductivity can be as high as 187 W/m·K. This order of magnitude higher conductivity results in more heat being applied to the water for evaporation from the heat source, thus the system doesn't cool and the air reaches a higher content of humidity.